<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
	<style>
		#box{
			width: 200px;
			height: 200px;
			background: red url() 0 0 no-repeat;
		}

		#box1{
			width: 200px;
			height: 200px;
			background: yellow url() 0 0 no-repeat;
		}
		#son{
			width: 100px;
			height: 100px;
			background: blue url() 0 0 no-repeat;
		}
	</style>
	<script>
		
		window.onload= function () {
			son = document.getElementById("son");
			son.parentNode.nextSibling.style.background="green";
		}

	</script>
</head>
<body>
	<div id="box">
		<div id="son" class="son">
			
		</div>
	</div>
	<div id="box1">
		
	</div>
</body>
</html>